1. Field of the Invention
The present invention pertains to a method of recovering petroleum fluids from an underground reservoir. More specifically, it pertains to a method of recovering petroleum fluids particularly suited for recovering petroleum fluids retained in the reservoir after the production zone is watered out.
2. Description of the Prior Art
After an oil field is abandoned, fifty-two percent of the original oil (on a national average) remains trapped in the reservoir. Before secondary and tertiary recovery methods were developed to recover additional petroleum fluids trapped in the oil-bearing formation after initial production, this percentage was even higher, e.g. seventy percent.
There are several secondary and tertiary recovery methods utilized today. One of these is called "water flooding". In water flooding, water is pumped into the formation and pushes some of the remaining oil sideways instead of upward. Although water flooding may double production from a reservoir, the water must be clean, treated with bacteriacides and slimacides to prevent plugging. In addition, scale inhibitors may be required. The cost of water flooding may exceed $1 per barrel of water injected and may require a volume of water equal to twenty times the volume of oil displaced thereby.
Another secondary or tertiary method of production involves the introduction of gas during primary production. The gas replaces the oil as it is produced, maintaining a fairly steady water/oil contact. The oil in the gaseous region trickles down to the production zone. This method is primarily used with light crudes and is considered to be a rather poor recovery method.
An outmoded and seldom used secondary recovery method is burning gas by a downhole heater at the well face and forcing the hot gases into heavy oil zones to thin the oil for better production.
In steam injection, steam is injected into the formation. As the steam condenses, it gives off heat which thins the oil resulting in greater production. This method is widely used for heavy crudes. It may be considered a modified hot water flood.
In another method, carbon dioxide is injected into a reservoir. The carbon dioxide is dissolved, increasing the volume of the oil by about thirteen percent and also thinning the oil. This method requires as much as 12,000 to 20,000 cubic feet of carbon dioxide per barrel of oil (5.6 cubic feet per barrel). Although ten to fifteen percent more oil is recovered, this method is used only moderately today.
In a method similar to carbon dioxide injection, exhaust combustion gases are injected into the reservoir. Exhaust gases contain carbon dioxide and about ninety percent nitrogen which has little use other than as a heat carrier. This method is seldom used today.
In another method, solvent, frequently heated, is injected into the reservoir. By this method, a mixture of solvent and oil is obtained which is less viscous than the oil. This method is used with crudes which are almost tars.
Another recovery method is fire flooding. In fire flooding, air is injected into the reservoir and the reservoir is burned. Approximately thirteen percent of the oil is burned to recover the rest. The weight of air pumped into the reservoir exceeds the weight of oil produced. Furthermore, the heat produced by fire flooding can melt the sand in the formation into glass. Acids formed by this method may also destroy well casings. Fire flooding is normally limited to a maximum depth of about 3000 feet. Both light and heavy ends of the oil are burned. The middle fraction is cracked or oxidized to organic acids.
A popular method of recovery today is the detergent flooding method. In this method, detergents and possibly caustic and/or sacrificial materials are introduced in a water flow. Under ideal conditions, three barrels of water and one pound of detergent will produce one barrel of oil. In a related method, caustic water may be injected into formation. Caustic reacts with naphthenic acids in the oils to form detergents in situ. In this method, clays may create a problem.
An expensive and seldom used method is polymer flooding. Since the viscosity of oil is much higher than water, water often fingers into production wells and once a clear channel is formed, little oil is formed. In polymer flooding, high molecular weight water soluble polymers are injected into the formation to give a bank ahead of the water flood. This equalizes viscosities and produces more of the by-passed oil.
The search continues for more efficient and less expensive methods of recovering oil.